1. Field of the Invention
The present invention relates to a control method and device for an optical filter.
2. Description of the Related Art
With an explosive increase in demand for data communication centered on the Internet traffic, high-capacity transmission and ultra long-haul transmission are desired in a backbone network. Further, since a variety of different services are offered to users, a highly reliable, flexible, and economical network is desired.
The high-capacity transmission and the ultra long-haul transmission are rapidly proceeding owing to the wavelength division multiplexing (WDM) technique and the optical amplifying technique, thereby allowing a reduction in transmission line cost. However, high-speed transmission and high-capacity transmission of signals increase the amount of information to be processed at network nodes, causing an increase in node cost and scale. Under such circumstances, the development of an optical add/drop multiplexer (OADM) and an optical cross-connect (OXC) is being pursued at a high pace, so as to reduce the cost and scale of the network nodes in such a manner that optical components are substituted for electronic circuits to perform a variety of processing on the basis of an optical path in the wavelength domain.
The OADM and the OXC employ many optical functional devices such as an optical switch having a function of switching on/off light, attenuating light, or switching between one input and n outputs, and an optical wavelength filter (optical filter) for selecting an optical signal according to wavelength. Of these optical functional devices, the optical filter is one of the key devices used for wavelength switching in the OXC and the OADM, wavelength separation at a receiving section, ASE (amplified spontaneous emission) noise cutting, etc.
In the OADM and the OXC, optical path switching is performed according to wavelength, so that the optical filter is required to have a wavelength tuning function of selecting a desired wavelength. With this wavelength tuning function, the transmission center wavelength of the optical filter can be tuned to coincide with a desired wavelength, thereby allowing the selection and separation on wavelength. In general, the wavelength of a transmitted light source has fluctuations, and the transmission center wavelength of the optical filter itself is also fluctuated by aged deterioration, environmental change, control error, etc. Therefore, it is necessary to perform tracking control for making the transmission center wavelength always coincide with a signal wavelength. For example, since even a slight deviation between the wavelength of transmitted light through the optical filter and the transmission center wavelength of the optical filter has a large effect on transmission characteristics or the like, a high-precision tracking technique is required. In the case of a notch filter, for example, the tracking control is performed by using an error signal obtained by extracting light having a desired wavelength and then monitoring the power of the light.
Further, an optical filter, e.g., a notch filter, for use in extracting an optical signal having a desired wavelength from a plurality of densely wavelength division multiplexed optical signals is required to have a filtering shape such that the adjacent channels with respect to the desired wavelength channel are not affected and that a sufficient rejection level can be obtained. In this case, the optical filter is generally configured by cascading a plurality of optical filter units for the purposes of improvement in rejection level and narrowing of a filtering band.
A generally considered tracking method for an optical filter, e.g., a notch filter, configured by cascading a plurality of optical filter units includes the steps of detecting monitor light from the optical filter unit on the first stage and controlling all the optical filter units by using the detected monitor light. However, the plural optical filter units cascaded have minute individual differences or the like, so that even when the same control signal is input to each optical filter unit, the transmission center wavelengths of all the optical filter units do not always completely coincide with each other in many cases. Accordingly, the filtering shape of the optical filter as a whole becomes different from that in the case where the transmission center wavelengths of all the optical filter units coincide with each other. If the tracking control is performed by using the monitor light obtained from the first-stage optical filter unit in this condition, the transmission center wavelength of the optical filter as a whole is deviated from the wavelength of an optical signal to be passed through the optical filter, because the transmission center wavelength of the first-stage optical filter unit is different from the transmission center wavelength of the optical filter.
The deviation of the transmission center wavelength of the optical filter from the wavelength of the optical signal causes a large deterioration in rejection characteristic. In the worst case due to this deviation, there is a possibility that an optical signal of another wavelength channel may be undesirably selected to cause a large transmission deterioration.